Phonegap - Extend a plugin for iOS

Question

I've been reading about how to create a new plugin for Phonegap and seems to get it by following this tutorial.

http://wiki.phonegap.com/w/page/36753496/How%20to%20Create%20a%20PhoneGap%20Plugin%20for%20iOS

However, I'm having a hard time grasping how to extend an existing plugin. (not creating a new one)
Been reading this tutorial.

http://hiediutley.com/2011/03/28/phonegap-tutorial-series-3-extending-the-phonegap-api/

I can't seem to get where I can possibly add line of codes to the .m file for example. In my XCode, I only see the .h files but not the .m file.
Or is there a better way to extend an api?

Thank you,
Tee